There are an enormous range of problems associated with steels that are all superficially designated as "corrosion". And there are hundreds if not thousands of different solutions to these various corrosion problems. These various types of corrosion each have different mechanisms and sometimes different consequences. Given the different mechanisms, the solution to one corrosion problem is generally not applicable to another. In other words, it is difficult to predict with any reasonable expectation of success whether a solution effective for one corrosion problem is likely to be effective for another, different corrosion problem.
The present invention is related to one specific type of corrosion--high-temperature hydrogen attack of carbon and low-alloy steels. The term "hydrogen attack" is well known in the art. For example, in the book, "Corrosion in the Petrochemical Industry" edited by L. Garverick (1994), it is defined on pp. 59:
"Hydrogen attack is a high-temperature form of hydrogen damage that occurs in carbon and low-alloy steels exposed to high-pressure hydrogen at high temperatures for extended time. Hydrogen enters the steel and reacts with carbon either in solution or as carbides to form methane gas; this may result in the formation of cracks and fissures or may simply decarburize the steel, resulting in a loss in strength of the alloy. This form of damage is temperature dependent, with a threshold temperature of approximately 200.degree. C. (400.degree. F.)." PA1 a) treating a carbon or low-alloy steel portion of a reactor system which is to be contacted with high pressure hydrogen, and optionally hydrocarbons, sulfur and oxygen compounds including water, with a metal component selected so that it produces an intermetallic surface diffusion barrier layer which reduces the rate of hydrogen permeation through the steel by a factor of at least 10; and PA1 b) passing high pressure hydrogen over said metal-treated steel at temperatures between about 400.degree. F. to 1050.degree. F. and at hydrogen pressures above 400 psig. PA1 (a) applying a metal plating, paint, cladding or other coating to a steel portion made of carbon or low-alloy steel that has been subjected to hydrogen attack conditions; and PA1 (b) forming an intermetallic, diffusion barrier layer on the steel surface by heating; thereby reducing the rate of hydrogen permeation through the steel portion by a factor of at least 10 compared to a steel portion without the barrier layer.
Hydrogen attack is a significant problem in petroleum refineries and chemical plants. This problem is compounded in that it is difficult to monitor or observe hydrogen attack by inspection of in-place equipment. Moreover, there is an induction period before hydrogen attack occurs. Yet, failure to replace equipment that is or has suffered hydrogen attack can lead to metallurgical failure, with hydrogen and/or hydrocarbons release. This can lead to fires and even explosions.
Hydrogen attack should not be confused with other types of corrosion caused by hydrogen in different environments and under different reaction conditions. For example, hydrogen embrittlement of steel is a totally different process. It is an low-temperature, low pressure, aqueous process that starts with proton (H+) adsorption and diffusion into the interstitial spaces between the iron molecules in the steel structure. This aqueous, cathodic corrosion changes the way the steel responds to stress; after embrittlement, the steel ductility is reduced, and it may fracture rather than bend. Some proposed solutions to the problem of aqueous hydrogen embrittlement are described in Chen et al, "The Use of Zinc and Tin Coatings and Chemical Additives for Preventing Hydrogen Embrittlement in Steel", Corrosion Prevention and Control, June 1993, pp. 71-4.
Another type of corrosion which is unrelated to hydrogen attack is carburization. Carburization occurs in high temperature hydrocarbon environments. In mechanism, carburization is almost the opposite of hydrogen attack. Carburization is the injection of carbon into the steel. This injected carbon forms surface metal carbides, which embrittle the steel. Some solutions to this carburization problem in low sulfur reforming are described in Heyse et al., WO 92/15653. Solutions to the carburization problem in other processes are described in WO 94/15898 and WO94/15896, both to Heyse et al. Among these solutions is the use of metallic tin coatings. However, the parts of commercial process equipment where carburization and metal dusting are a concern are designed and constructed of materials such as high alloy or stainless steel. Here hydrogen attack is not a problem.
Currently, there are a wide variety of petroleum-related processes that have equipment made of carbon and low-alloy steels. Some of this in-place metallurgy is operated under conditions that can potentially result in high-temperature hydrogen attack of the steel. These processes include, for example, hydrotreating, hydrofining, hydrocracking and hydrogen production. Desulfurization and/or denitrification of hydrocarbon feeds is often the process objective. Hydrogen attack is most problematic in the hot loop, i.e., in reactors, steam generators, heat exchangers and associated piping, since both the rate of hydrogen diffusion though the steel and the thermodynamic driving force for methane formation (and therefore the rate of hydrogen attack) increase with increasing temperature.
In many instances the in-place metallurgy, that is, the carbon or low-alloy steel, was originally expected to operate safely at typical process conditions, that is, it was expected that hydrogen attack would not occur. However, it has been shown that the susceptibility of certain low-alloy steels to hydrogen attack is greater than previously believed. Today, the concerns associated with hydrogen attack of the steel have limited the operating conditions and necessitates regular inspections of the steel.
There are few commercial solutions to the problem of hydrogen attack in existing equipment. One solution is to operate at reduced severity (lower) and suffer whatever yield losses or reduced throughput is required. Another solution is to replace the carbon or low-alloy steel with a steel that is not susceptible to hydrogen attack at the reaction conditions. For example, a higher alloy steel or a stainless steel containing chromium and optionally nickel can be used. Replacing the steel is a major undertaking and can be quite costly.
As described above, a practical, effective and inexpensive solution to the hydrogen attack problem--especially for carbon and low-alloy steels already in place and in use--has long been needed. One object of the present invention is to provide such a solution.